1. Field of the Invention
The invention relates to the field of aligning the streets of a semiconductor wafer for sawing, scribing or other operations.
2. Prior Art
During the fabrication of integrated circuits, most often a plurality of identical circuits are simultaneously formed on a circular semiconductor wafer. The circuits are separated from one another by "streets"; the streets generally form an orthogonal grid on the wafer. After fabrication, the circuits are separated into "chips" at the streets by sawing the wafer or by scribing the wafer and breaking it into chips.
When a wafer saw or similar tool is employed to saw or scribe the wafer, the wafer is first secured on a working surface such as a vacuum chuck. In the case of a wafer saw, the saw blade typically moves only in the Y-direction and the chuck is moved in the X-direction to saw the wafer. Before the wafer can be cut, it must first be aligned with the blade.
In most wafer saws, the vacuum chuck moves in the X, Z and .theta.-directions and the blade and microscope assembly (hereinafter referred to as the "blade assembly") moves in the Y-direction. The operator generally manipulates the Y- and .theta.-direction controls to align a street with a cutting path. Since the exact position of the wafer on the chuck is unknown, this alignment is time consuming. Specifically, since the center of rotation of the chuck is not aligned with the center of the street grid, rotation of the chuck results in a translation of the streets as well as rotation.
In one commonly employed method, a street is aligned with the cutting path by movement in the .theta.- and Y-directions. The operator first controls the rotational movement of the wafer chuck. However, since a street may not be in line with the reticle of the microscope, the operator must move the blade assembly in the Y-direction, and then return to the rotational control to obtain this parallelism. Typically, the operator must alternate between the .theta. and Y-direction controls a number of times to align a single street with the cutting path since rotation of the chuck results in a translation of the street.
In another method, the alignment procedure begins with the reticle in the center of the chuck. In still another method, a split field microscope is employed. In both these latter methods, the operator must coordinate both the rotational and Y-direction movement in order to align the wafer.
As will be seen with the present invention, the operator need only control a single aligning, motion control switch and need not alternate between a Y-direction motion control switch and a rotational motion control switch to align the streets of the wafer.